Question

Table “argument_user”

+-------------+----------+
| argument    | user_id  |
+-------------+----------+
| bike        | 1        |
| flower      | 2        |
| web         | 1        |
| web         | 2        |
| nature      | 1        |
| nature      | 2        |
| nature      | 3        |
+-------------+----------+

I want get ONLY the arguments in common with user 1 and 2 >> “web” (without “nature”)

+-------------+
| web         |
+-------------+

Now I’m using this query, but it’s wrong because it returns me “web” and also “house”.

select argument from argument_user 
where user_id in ('1','2') 
group by argument having count(distinct(user_id)) = 2

+-------------+
| web         |
| nature      |
+-------------+

I thought to do a join with the same table, but i think it’s too heavy when the users will be more! don’t you?
So, something light to use with a lot of “user_id”.

Answer 1

Using a couple of JOINs

SELECT *
FROM argument_user a
INNER JOIN argument_user b
ON a.argument = b.argument
LEFT OUTER JOIN argument_user c
ON a.argument = c.argument
AND c.user_id NOT IN (1,2)
WHERE a.user_id = 1
AND b.user_id = 2
AND c.user_id IS NULL